The present invention is related to control track recording, and particularly to an improved audio or control track record head drive circuit.
In the field of videotape recorders, and particularly when recording television via a helican scan format, it is standard practice to record a control track along with selected audio tracks on the tape. The control track provides tape position signals for the capstan servo, the reel servo and for the tape timer circuit, and also provides information for the identification and phase control of color frame/sync sequences. In SMPTE or EBU type C helical recording formats, audio tracks identified as A1 and A2 are recorded adjacent the video along the top edge of the tape. The control track is longitudinally recorded adjacent the helically recorded video, along the bottom edge of the tape. A third audio track A3 is recorded adjacent to the control track along the bottom-most edge of the tape. The spacing between track A3 and the control track is twice as much as is the spacing between the tracks A1 and A2.
The SMPTE and EBU systems offer three format options which affect the tracks recorded near the tape bottom edge. The first or basic NTSC or PAL format option has no sync track, whereby only the control track and the audio track A3 are recorded. A second NTSC or PAL format option includes the sync track, which is recorded via helical sync heads in the space between the control and A3 tracks. Thus information occurring during vertical interval drop-out time is recorded and played back by the sync track heads. A third format option for PAL with EBU audio track A4, allows PAL/SECAM machines equipped for EBU audio, to record the fourth audio track in the space between the control track and the track A3 which is reserved for the sync information in the second option.
Different television recording formats use different schemes for recording the control track. The SMPTE type C format records a saturated control track which has a square wave fundamental. The EBU type C format uses a biased pulse signal. The former scheme provides a control track which saturates the tape in opposite polarities with rather large amplitude 30 Hertz (Hz) signals. The latter scheme uses less energy, and eliminates the low frequencies associated with the saturated signal. This minimizes crosstalk between adjacent channels, while making the addition of a high quality audio A4 track practial in the EBU system.
In both systems there exists a problem of crosstalk between the tracks when recording the control track and attempting to read out the audio track A3. In the SMPTE format wherein the control track is recorded as a saturated signal of relatively large amplitude, the problem of crosstalk must be taken into account, even though the spacing is relatively large between the control track and the audio track A3. In the EBU format, the resulting decrease in space between the control track and the audio track A4, requires precautions to minimize crosstalk between the tracks when recording the control track and playing back the track A4, even though the EBU control track signal uses a biased small pulse signal.
Accordingly, in recording a control track, there are various conditions and associated problems which must be dealt with. For example, the control track drive circuit should properly record the control signal to allow its efficient and faithful reproduction upon playback, and should minimize or delete entirely any objectionable level of crosstalk between the control track and the adjacent audio track. Further, the drive circuit should not be affected by a change in inductive reactance of the driven head, which may vary between heads as much as .+-.20%. In high qualilty audio head applications, the drive circuit should not generate any phase changes in the response characteristics of the recorded audio signal.
There also are various practical considerations which should be taken into account in the design of a control track, or an audio track, head drive circuit. Typical of such considerations are that the circuit be relatively simple in design, and require as few "tweaking" adjustments as possible when performing the final system test of the apparatus. Adjustments in a driver circuit generally are interreactive and therefore require excessive and cumbersome tweaking in order to properly adjust them to the correct proportions.
In keeping with the above considerations, various circuits have been developed in the art to provide control track head driver circuits which provide the preferred control track parameters as well as the preferred circuit features. Typical of such control track drive circuit is that used in the video production recorder VPR-2B manufactured by Ampex Corporation, Redwood City, California, which employs an "interrupted" bias control track drive circuit for the EBU format, and a now-biased saturated squarewave drive circuit for the SMPTE format.
The former circuit circumvents the problems usually associated with conventional additive bias schemes which employ, for example, a bias transformer, separate feeds for bias drive and record drive, and two level potentiometers which require careful tweaking adjustments to provide the correct drive proportions. It follows that the system is readily simple to set up with a minimum of tweaking adjustments.
However, the scheme used to bias the control track record signal, employs the interrupted bias/record signal of previous mention, which causes an increase in interchannel crosstalk. The bias scheme employs a squarewave oscillator directly coupled to the drivers of the record head, which generates a signal at a bias rate of, for example, 125 kiloHertz (kHz). After every eight microseconds (us) interval, the bias is interrupted for 125 us to provide a record signal of the same amplitude as the bias signal and of alternating polarity. The result is a continuous squarewave formed of eight microseconds of AC bias, interspersed with 125 us record signals of alternate polarity. This composite signal is recorded on the tape as the control track signal, in the EBU system. The playback system is fairly insensitive to the very fast bias transitions, but is very sensitive to the slow transistions and thereby detects the latter as the control track signal. However, the actual recorded signal has a very high record current component which, in turn, causes excessive crosstalk between the control track and the audio tracks A3 and A4.
The present invention provides a drive circuit for a control track record head, which overcomes the various disadvantages of the drive circuits of previous mention. To this end, a constant current source is combined with a fixed AC bias circuit, to provide an additive bias head driver circuit which derives all the advantages, but none of the disadvantages, of present additive or "interrupted" bias schemes. More particularly, the circuit does not require a coupling transformer and thus circumvents the expense, the unreliability and the associated tweaking adjustments caused by its use. The constant current source receives a fixed bias from an AC filter bias means, along with the selected record signal, and supplies a constant current biased drive signal to the control track record head. Inverting amplifier means also are coupled to the record head. The combined configuration of the constant current source and the inverting amplifier means defines a voltage doubler bridge circuit, which supplies the head with twice the driving current which would be provided by a conventional single-ended configuration.
Accordingly, it is an object of the invention to drive a record head using fixed AC bias and fixed recording level signals.
A further object is to provide an improved circuit for driving a record head which does not require cumbersome tweaking adjustments.
Another object is to provide a reactive load driver circuit which is unaffected by differences in the load inductive reactance.
Still another object is to provide an improved driver circuit which requires half the usually required voltage.
A still further object is to provide an improved driver circuit for recording high quality audio signals as well as control track signals.